1. Field of the Invention
The invention relates to an armature winding of a rotating electrical machine, which is configured to prevent generation of corona discharge by reducing a potential difference between adjacent coil pieces.
2. Description of the Related Art
A conventional armature winding of a rotating electrical machine has a structure shown in FIG. 33.
FIG. 33 is a sectional view showing an example of an armature winding placed in a slot of an armature core of a rotating electrical machine.
In FIG. 33, a reference number 12 denotes an armature core formed by stacking a laminated iron plate. An armature winding 14 is housed in a slot 13 provided in the circumference of the armature core 12.
The armature winding 14 is provided in two layers of upper coil pieces 15 close to the slot opening, and lower coil pieces 16 close to the slot bottom, and the outer circumference is covered by a main insulating layer 24.
An insulating plate 10a is provided at the slot bottom, and an insulating partition plate 10b is provided between the upper coil piece 15 and lower coil piece 16. A wedge 9 is inserted into the slot opening through an insulating plate 10c. 
In a large-capacity rotating electrical machine, the machine capacity is increased by increasing a generated voltage by connecting the upper and lower coil pieces 15 and 16 of the armature winding 14 in series.
As a voltage of the armature winding 14 is increased, the thickness of the main insulating layer 24 of the armature winding is increased to withstand the voltage. As a result, a cross section of a conductor is decreased, a current density is increased, and a loss is increased. Thus, the armature winding is divided into multiple parallel circuits, the armature winding voltage is decreased without changing the machine capacity, and the thickness of the main insulating layer 24 is decreased, thereby decreasing the loss and increasing the cooling capacity.
First, a first example of a conventional armature winding is described.
FIG. 34 is a developed perspective view of one phase of a 2-pole 3-phase 66-slot armature winding having two parallel circuits. FIG. 35 is a developed perspective view of three phases of the same armature winding shown in FIG. 34. The other two phases not shown in FIG. 34 are obtained by displacing the configuration of the shown armature winding by 120° and 240°, respectively.
As shown in FIG. 34, in the armature winding 14, eleven slots are occupied by each of the upper and lower coil pieces per one phase belt.
The phase belt mentioned here means a part of a winding, which forms one same phase by dividing each of three phases into multiple parts, housing upper and lower coil pieces in two layers in multiple slots of an assigned laminated iron core (an armature core), and sequentially connecting them in series.
As shown in FIG. 34, the armature winding 14 of each phase has the upper coil piece 15 (15a, 15b) housed in a slot close to an opening, and the lower coil piece (16a, 16b) housed in a slot close to a bottom. The ends of the upper and lower coil pieces 15 and 16 are connected in series at a connection side coil end 19a connected to a lead-out portion of the winding, and at a counter-connection side coil end 19b that is axially opposite and not connected to a lead-out portion of the winding.
Further, the armature winding 14 has a first phase belt 17, in which each of the upper and lower coil pieces 15 and 16 are housed in eleven slots 13 (1st to 11th slots, and 28th to 38th slots) provided in the armature core 12, and a second phase belt 18, in which each of the upper and lower coil pieces 15 and 16 are housed in eleven slots 13 (34th to 44th slots, and 61st to 5th slots).
The armature winding 14 of each phase has two parallel circuits, which are indicated by a solid line and a broken line.
In the first and second phase belts 17 and 18, the upper coil piece 15 is connected to the corresponding lower coil piece 16 separated by a fixed pitch, at the connection side and counter-connection side coil ends 19a and 19b, thereby forming two parallel circuits. The parallel circuits are connected in parallel through a lead-out connection conductor 21 provided at the connection side coil end 19a, thereby forming the armature winding 14.
In FIG. 34, on the counter-connection side, an upper coil piece placed in the 1st slot (the 1st upper coil piece) is connected to a lower coil piece placed in the 28th slot (the 28th lower coil piece), and a coil pitch is 27. On the counter-connection side, an upper coil piece placed in the 11th slot (the 11th upper coil piece) is connected to a lower coil piece placed in the 37th slot (the 37th lower coil piece), and a coil pitch is 26.
In the armature winding shown in FIG. 34, a winding pitch means a pitch between the slot 13 housing the upper coil piece 15a of the first phase belt 17 and the slot 13 housing lower coil pieces, for example, and the winding pitch in FIG. 34 is 27. In other words, the upper and lower coil pieces are connected to have a coil pitch one less than a winding pitch at the connection side coil end, and a coil pitch equal to a winding pitch at the counter-connection side coil end.
When the number of parallel circuits of each phase belt is less than the number of poles, as seen in Patent document 1 (FIG. 2), for example, a lead-out portion is usually provided at the end of a phase belt, and a conductor of the lead-out portion is connected to a coil piece at the phase belt end. For example, in FIG. 34, at the connection side coil end 19a of the first phase belt 17, the lead-out connection conductor 21 connected to the output terminal 22 is connected to the 1st coil piece of the upper coil piece 15a, and the 38th coil piece of the lower coil piece 16a is connected to the lead-out connection conductor 21, and further connected to a neutral terminal 23.
In the second phase belt 18, at the connection side coil end 19a, the lead-out connection conductor 21 connected to the output terminal 22 is connected to the 5th coil piece of the lower coil piece 16a, and the 34th coil piece of the upper coil piece 15b is connected to the lead-out connection conductor 21, and further connected to the neutral terminal 23.
In the following description, the centers of magnetic pole Pa and Pb are defined as shown in FIG. 34, and in the first phase belt, the 1st upper coil piece is called an upper coil piece positioned at the phase belt end far from the magnetic pole center, and the 11th upper coil piece is called an upper coil piece positioned at the phase belt end close to the magnetic pole center. Concerning the lower coil piece, the 28th lower coil piece is called a lower coil piece positioned at the phase belt end close to the magnetic pole center, and the 38th lower coil piece is called a lower coil piece positioned at the phase belt end far from the magnetic pole center.
Therefore, in normal connection, the first upper coil piece that is an upper coil piece positioned at the phase belt end far from the magnetic pole center is connected to the 28th lower coil piece that is a lower coil piece positioned at the phase belt end close to the magnetic pole center.
In the armature winding 14, coil pieces are sequentially wound from the left side to right side in FIG. 34, namely from the 1st upper coil piece to 28th lower coil piece, 2nd upper coil piece, and 29th lower coil piece. Hereinafter, this will be described as coil pieces wound so that upper coil pieces are positioned close to the magnetic pole center.
Similarly, in the second phase belt, the 5th lower coil piece, 4th upper coil piece, 4th lower coil piece, and 43rd upper coil piece coil pieces are wound so that lower coil pieces are positioned close to the magnetic pole center.
In the following description, a phase comprising the 1st to 11th upper coil pieces is called a V-phase, a phase comprising the 12th to 22nd upper coil pieces is called a U-phase, and a phase comprising the 23rd to upper coil pieces is called a W-phase. This will be repeated in the following coil pieces. It is of course no problem if the configuration and sequence of the V, U and W phases are changed.
Next, a potential difference between adjacent coil pieces is explained. Here, the potential difference is indicated by p.u. as a ratio to an induced voltage in one phase.
Regarding the coil pieces at the phase belt end in FIG. 35, the potential of the upper coil piece placed in the 23rd slot (the 23rd upper coil piece) at the connection side end is 1[PU], and the potential of the upper coil piece placed in the adjacent 22nd slot (the 22nd upper coil piece) is 10/11 [PU].
FIG. 36 shows a vector indicating an example of the potential difference between the adjacent coil pieces in FIG. 35. The potential difference between the 23rd upper coil piece and 22nd upper coil piece is 1.654 [PU] considering a phase difference.
On the other hand, as the upper coil piece placed in the 12th slot (the 12th upper coil piece) is connected to the neutral terminal 23, the potential of the 12th upper coil piece at the connection side end is 0 [PU] at the connection side end, and the potential of the upper coil piece placed in the adjacent 11th slot is 1/11 [PU]. As indicated by the vector in FIG. 36, the potential difference between the 12th upper coil piece and 11th upper coil piece is 1/11=0.091 [PU].
On the other hand, between coil pieces of the same phase, the potential difference between any coil pieces is 1/11=0.091 [PU].
FIG. 37 shows the potential differences between adjacent coil pieces, particularly upper coil pieces, at the connection side coil end obtained as described above. The phases shown in legends indicate the phases of coil pieces to which smaller numbers are given.
As shown in FIG. 37, in the above-described 2-pole 3-phase 66-slot armature winding having two parallel circuits, a maximum potential difference between adjacent coil pieces is 1.654 [PU] in the boundary of phase belts.
Next, a second example of a conventional armature winding is explained.
FIG. 38 is a developed perspective view of one phase of a 2-pole 3-phase 66-slot armature winding having one parallel circuit. The number of slots occupied by each of the upper and lower coil pieces per one phase belt is 11, as in the armature winding shown in FIG. 34.
In FIG. 38, the winding pitch is 27, which is the same as the coil pitch 27 at the counter-connection side coil end 19b, and the coil pitch 26 at the connection side coil end 19a is one less than the winding pitch.
In FIG. 38, as in FIG. 34, a coil piece at the phase belt end is connected to a lead-out connection conductor. For example, in FIG. 34, at the connection side coil end 19a of the first phase belt 17, the 1st coil piece of the upper coil piece 15a is connected to the lead-out connection conductor 21, and further connected to the output terminal 22, and at the connection side coil end 19a of the second phase belt 18, the 34th coil piece of the upper coil piece 15b is connected to the lead-out connection conductor 21, and further connected to the neutral terminal 23.
The 38th coil piece of the lower coil piece 16a in the first phase belt 17 and the 5th coil piece of the lower coil piece 16b in the second phase belt 18 are connected by a connection side jumper wire 20a. 
Next, the potential difference between adjacent coil pieces will be explained. For example, regarding the coil pieces at the phase belt end in FIG. 38, the potential of the 44th upper coil piece at the connection side end is 10/22 [PU], and the potential of the not-shown adjacent 45th upper coil piece is 1 [PU]. The potential difference between the 44th upper coil piece and 45th upper coil piece is 1.289 [PU] considering a phase difference of 120°.
FIG. 39 shows a potential difference between adjacent coil pieces at a connection side coil end. In the second example of a conventional armature winding, a maximum potential difference between adjacent coil pieces is 1.289 [PU] in the boundary of phase belts.
Next, a third example of a conventional armature winding is explained.
In a large-capacity rotating electrical machine using an indirect cooling system, in particular, it is common to increase a cooling cycle of an armature winding by increasing the number of slots in an armature core. Thus, an armature winding having three or more parallel circuits is required. When the number of parallel circuits is more than the number of poles, as in a 2-pole 3-parallel-circuit armature winding, it is impossible to completely equate the voltage generated in each parallel circuit, and a circulation current is generated between the parallel circuits, increasing a loss in the armature winding.
To decrease a loss caused by a circulation current, it is necessary to minimize the imbalance in the voltages generated in the parallel circuits. Therefore, it is necessary to take special care in the arrangement of coil pieces in each parallel circuit in each phase belt.
FIG. 40 is a developed perspective view of one phase of an armature winding described in Patent document 4. This is a 2-pole 3-phase armature winding having 72 slots in an armature core. The number of slots per one phase belt occupied by each of the upper and lower coil pieces is 12.
The armature winding comprises three parallel circuits, 1 to 3, indicated by three kinds of line in FIG. 40. The parallel circuits of twelve upper coil pieces 15a and lower coil pieces 16a, constituting a first phase belt 17, are numbered 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3 sequentially from the left side, and similarly, the parallel circuits of twelve upper coil pieces 15b and lower coil pieces 16b, consisting a second phase belt, are numbered 3, 2, 1, 3, 2, 1, 3, 2, 1, 3, 2, 1 sequentially from the left side, thereby decreasing a voltage deviation (an absolute value of deviation from an average phase voltage) in the parallel circuits and a phase deviation (a phase angle deviation from an average phase voltage) in the parallel circuits.
To realize the above connection, in the armature winding 14 shown in FIG. 40, the lead-out ends of the parallel circuits of the first and second phase belts 17 and 18 are connected by the lead-out connection conductor 21.
The potential difference between adjacent coil pieces is 1.231 [PU] at maximum in the boundary of the phase belts, between the 1st upper coil piece and not-shown 72nd upper coil piece, for example.
Next, a fourth example of a conventional armature winding is explained.
Patent document 5 describes an improvement to simplify the structure of an armature winding of a rotating electrical machine shown in the third example.
FIG. 41 is a developed perspective view of one phase of an armature winding improved by the method described in patent document 5. This is a 2-pole 3-phase armature winding having 72 slots in an armature core. The number of slots per one phase belt occupied by each of the upper and lower coil pieces is 12.
The armature winding comprises three parallel circuits, 1 to 3, indicated by three kinds of line in FIG. 40. The parallel circuits of twelve upper coil pieces 15a and lower coil pieces 16a, consisting a first phase belt 17, are numbered 1, 2, 1, 2, 1, 1, 2, 1, 1, 2, 1 sequentially from the left side. Similarly, the parallel circuits of twelve upper coil pieces 15b and lower coil pieces 16b, consisting a second phase belt 18, are numbered 3, 2, 3, 3, 2, 3, 3, 2, 3, 3, 2, 3 sequentially from the left side. The parallel circuit 3 of the first phase belt and parallel circuit 1 of the second phase belt, which are electrically equivalent, are interchanged so that the parallel circuits 1 and 3 are placed in the same phase belt.
To realize the above connection, in the armature winding 14 shown in FIG. 41, the connection side lead-out end 19a is expanded by a 2-phase jumper wire 20a, and the lead-out terminals of the parallel circuits of the first and second phase belts 17 and 18 are connected by the lead-out connection conductor 21.
The potential difference between adjacent coil pieces is 1.625 [PU] at maximum in the boundary of the phase belts, between the 1st upper coil piece and not-shown 72nd upper coil piece, for example.
When the voltage of the armature winding is increased, the potential difference between adjacent coil pieces is increased. Particularly, in the above-described example of a conventional armature winding, the potential difference between adjacent coil pieces in the boundary of phase belts is increased, and corona discharge may occur during operation at the coil end in the boundary of phase belts.
Especially, in a rotating electrical machine using an indirect cooling system to cool an armature winding from the outside of a main insulating layer, when non-pressurized air is used as a coolant, corona discharge is more likely to occur than when using hydrogen as the coolant, the insulation is deteriorated, and the operation of a rotating electrical machine may become unstable. This becomes a problem when increasing the voltage and capacity of a rotating electrical machine.
In an ideal state, corona discharge occurs in an electric field of about 3 kV/mm in air at a room temperature and atmospheric pressure, but actually, corona discharge may occur in a lower electric field, depending on the surface conditions of a charged material.
To prevent corona discharge between coil pieces in the boundary of phase belts, patent document 1 improves corona resistance by winding a mica tape or sheet around a coil end, or the outside of a corona prevention layer.
Further, even if electric discharge occurs between coil ends, the corona resistant mica prevents corona discharge caused by the electric discharge, or decreases the insulation damage.
The invention described in the patent document 1 decreases the insulation damage without changing the coil shape, whereas a tape or sheet is further wound around the outside of an insulated coil. This deteriorates heat transfer in the insulating material, and coil cooling effect, causing localized overheating. Further, the size, including the insulating material, is increased, the maintenance workability is lowered, and the armature may be accidentally damaged by maintenance.
In patent document 2, two kinds of coil groups with different directions of induced magnetic flux are prepared and combined to decrease a potential difference between coil pieces.
However, in the configuration described in patent document 2, the coil end structure is restricted, and the coil group structure is complicated. The configuration is not suitable for a large-capacity rotating electrical machine such as a large turbine generator for thermal power generation.
Further, patent document 3 describes a 3-phase 4-pole 72-slot armature winding having three parallel circuits, in which coil pieces other than those at the ends of a phase belt are connected to a lead-out portion, and the voltage imbalance between the parallel circuits is reduced, and the potential difference between adjacent coils in the other phase is decreased on the connection side.
However, patent document 3 aims at reducing the voltage imbalance between parallel circuits, and describes a winding method with specific numbers of poles, slots and parallel circuits. It does not decrease the potential difference from the coil pieces of the adjacent other phase, while keeping the same state as in conventional examples with respect to the voltage imbalance between parallel circuits.
It is possible to provide a larger space between coil pieces to increase the distance between the coil pieces of different phases at a coil end. However, this increases the size of a rotating electrical machine, increases the amount of material, and the increased weight restricts transportation and installation of the machine. It is also possible to provide a larger space between coil pieces by deforming a coil end after assembling. However, this may damage the insulation when deforming the coils, generates an unbalanced electromagnetic force, causing a problem in the reliability of a rotating electrical machine.
It is necessary to increase a line current or a terminal voltage to increase the capacity of a rotating electrical machine. A line current is restricted by a temperature increase, and the size and weight of a coil doctor. Therefore, it is necessary to increase a terminal voltage. For this purpose, it is necessary to restrict corona discharge of an armature winding as described above.